C-type lectins are a family of Ca -dependent carbohydrate-binding proteins that undergo a reversible conformational change that renders them unable to bind Ca , hence carbohydrates, at mildly acid pH. In order to understand this structural transition, we have crystallized the Ca -free form of the lectin domain of mannose-binding protein C. We have prepared this protein with selenomethionine substituted for methionine, and we propose to solve the structure by multiwavelength anomalous dispersion phasing of the selenomethionyl protein, using the tuneable 1-5AD beamline to collect the data.